The present invention relates to a vari-focal (variable focal length) camera, especially to a method and apparatus for driving the taking lens system of a vari-focal camera.
Vari-focal cameras are in widespread use and have a taking lens system that is variable in focal length, for example, between a short focal length position such as a wide-angle position and a long focal length position such as a telephoto position by moving the lens barrel along the optical axis. The lens barrel is driven by a motor upon operation of a switch, so that the vari-focal camera is easy to operate.
Such a vari-focal taking lens system comprises a movable lens group and a stationary lens group which are mounted in a movable lens barrel and a stationary lens barrel, respectively. The focal length is changed by moving the movable lens barrel axially back and forth relative to the stationary lens barrel, using a motor. Meanwhile, the taking lens system must be stopped precisely at a designated predetermined position such as the wide-angle or telephoto position. Therefore, it is necessary to detect the lens position. An encoder has been generally used for this purpose, and the motor is stopped when it is detected by the encoder that the taking lens system is positioned in a predetermined position.
The motor is generally driven by DC current, so that the motor is not able to stop immediately after being disconnected from the DC current, and may overrun the predetermined position. For this reason, it is conventional to provide a dead zone such as an idling portion in a power transmission mechanism for transmitting rotational power of the motor to the taking lens system so that the overrun of the motor may be absorbed in the dead zone. In this way, the taking lens system is prevented from moving beyond the predetermined position even when the motor overruns. Furthermore, in order to prevent overrunning, the motor is rotated alternately in opposite directions about a position wherein the encoder detects the designated position of the taking lens system, thereby to stop the taking lens system in the designated position after moving it an appropriate number of times back and forth through that position.
On the other hand, in such a taking lens system as described above, even when the encoder indicates that the taking lens system has stopped in the predetermined position, the actual stopped position thereof can be slightly different when stopping after moving in one direction compared with stopping after moving in the opposite direction. This is because a lens driving mechanism including male and female screw threads, one of which is rotated to move the movable lens barrel, has backlash. The same problem also arises in a vari-focal camera wherein the taking lens system is moved by a combination of a pin and a cam groove provided in a movable lens barrel and a stationary lens barrel because of play between the pin and cam groove. Therefore, in order to stop the taking lens system each time precisely at a predetermined position, it is necessary to stop the movable lens barrel at the predetermined position always after moving finally in a predetermined direction. That is, if the lens barrel is predetermined to stop at a final position always after moving forwardly, then when the lens barrel must be retracted to be in a designated position, the lens barrel must be further moved rearwardly beyond the designated position and then moved forwardly toward the designated position.
However, a power transmission mechanism having such a dead zone as described above is complicated in construction and must be accurately constructed in order to absorb with precision the overrun of the motor.
For such a lens driving device wherein the taking lens system is moved repeatedly back and forth before being stopped at a predetermined position, a complex control system is necessary.
Additionally, if the movable lens barrel is accidentally touched or pulled out after being stopped at a predetermined position before the depression of the shutter release button, the taking lens system may be displaced from the predetermined position. In such a case, the photographer cannot detect the displacement, so that the pictures taken in this condition may be out of focus. Providing a lock mechanism for securely maintaining the lens barrel in the predetermined position would make the camera cumbersome to operate.